test: add comprehensive coverage for canonical URL filter

- Verify clean URL passthrough behavior
- Test query parameter removal functionality
- Test fragment removal functionality
- Test combined query params and fragments handling
- Ensure graceful degradation for malformed URLs

bskyweb/cmd/bskyweb/filters_test.go

@@ -0,0 +1,61 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/flosch/pongo2/v6"
+)
+
+func TestCanonicalFilter(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "clean URL",
+			input:    "https://bsky.app/profile/user",
+			expected: "https://bsky.app/profile/user",
+		},
+		{
+			name:     "URL with query params",
+			input:    "https://bsky.app/profile/user?utm_source=test",
+			expected: "https://bsky.app/profile/user",
+		},
+		{
+			name:     "URL with multiple params",
+			input:    "https://bsky.app/profile/user?utm_source=twitter&utm_campaign=test",
+			expected: "https://bsky.app/profile/user",
+		},
+		{
+			name:     "URL with fragment",
+			input:    "https://bsky.app/profile/user#section",
+			expected: "https://bsky.app/profile/user",
+		},
+		{
+			name:     "URL with both params and fragment",
+			input:    "https://bsky.app/profile/user?param=1#section",
+			expected: "https://bsky.app/profile/user",
+		},
+		{
+			name:     "malformed URL",
+			input:    "not-a-url",
+			expected: "not-a-url", // Should return original on error
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			inputValue := pongo2.AsValue(tt.input)
+			result, err := filterCanonical(inputValue, nil)
+			if err != nil {
+				t.Errorf("filterCanonical() error = %v", err)
+				return
+			}
+
+			if result.String() != tt.expected {
+				t.Errorf("filterCanonical() = %v, want %v", result.String(), tt.expected)
+			}
+		})
+	}
+}